Cellulose derivatives, such as cellulose ethers are conventionally prepared in two stages, i.e. (1) an alkalization stage wherein an alkali metal hydroxide is reacted with cellulose to prepare an alkali cellulose, and (2) a derivatization stage, preferably an etherification stage, wherein a derivatizing agent, such as an etherifying agent is reacted with the alkali cellulose to form the cellulose derivative. Dispersing agents or solvents are sometimes added to one or both stages to obtain better mixing. Typically cellulose in finely divided state is reacted in a first stage with an alkaline solution, e.g. an alkali metal hydroxide, the alkaline solution generally being sprayed upon the cellulose fiber and reacted therewith in an alkalization reaction to form the alkali cellulose. Preferably the alkali cellulose is reacted in a second stage with an etherifying agent in an etherification reaction to form the cellulose ether.
In the alkalization reaction the reactor is vigorously stirred, e.g. in a heterogeneous reaction medium, to mix the alkali metal hydroxide and cellulose as uniformly as possible, and generally the reaction is carried out at low, often ambient, temperature.
Cellulose reactions take place under heterogeneous conditions. The accessibility of the anhydroglucose units (AGU's) therefore influences the reactivity of the cellulose in the various reactions to a large degree. It has been known for a long time that the reactivity of the cellulose is substantially improved or even made possible by an activation process with a sodium hydroxide solution. Here, the aim of the activation is to increase the reaction rate or the maximum achievable degree of substitution in the subsequent reaction of the cellulose and to achieve a more uniform substituent distribution and complete accessibility of the anhydroglucose units.
As mentioned, the most frequently used activating agent is a sodium hydroxide solution which, being an intrafibrillar swelling agent, causes structural change of the fibrillar units of the cellulose. In addition, apart from the structural loosening of the cellulose, the presence of hydroxyl ions is required in numerous cellulose reactions, such as, for example, the formation of carboxymethylcelluloses, methylcelluloses, hydroxyalkylcelluloses, hydroxyalkylmethylcelluloses or cellulose xanthogenates. Uniform alkalization is highly desired for the product quality strived for.
Methylcellulose and its mixed ethers comprising methoxyl groups are typically prepared in a multistage process. In the first stage, the cellulose used is ground to a desired particle size spectrum. In the second stage, the ground cellulose is intimately mixed with an aqueous solution, preferably a concentrated aqueous solution, of an alkali metal hydroxide, in particular sodium hydroxide, in a mixer and activated to produce alkali metal cellulose. The known processes are spray alkalization in a suitable mixing unit in which the ground cellulose is sprayed with an alkali metal hydroxide solution. In a slurry process, the ground cellulose is suspended in a suspension medium and the alkali metal hydroxide is then added. In a slurry alkalization process, the cellulose is suspended in a sodium hydroxide solution and then passed through screw presses or sieve drum presses to remove excessive caustic soda. In the third stage, a heterogeneous reaction is effected with an alkyl halide to be added, such as, for example, methyl chloride or ethyl chloride, and with a hydroxyalkylating agent, such as ethylene oxide and/or propylene oxide and/or butylene oxide. Further process stages may comprise the purification of the cellulose ether, grinding and/or drying.
In the industrial production of MC (methyl cellulose) and MHAC (methyl hydroxyalkyl cellulose) the alkalization, and in particular the etherification with etherifying agents, such as, for example, methyl chloride, ethyl chloride, ethylene oxide, propylene oxide and/or butylene oxide, comprises exothermic reaction stages with considerable generation of heat. Since suspension media, such as, for example, dimethyl ether and/or methyl chloride are usually used in a slurry process, the increase in temperature is associated with a simultaneous pressure increase. Because the alkalization usually takes place at atmospheric pressure or slightly superatmospheric pressure, the reaction can therefore be divided into a low pressure (alkalization) and high pressure (etherification) process sequence.
For a very wide range of fields of use, cellulose ethers having different degrees of substitution are prepared. The alkyl substitution is generally described by the DS in cellulose ether chemistry. The DS is the average number of substituted OH groups per anhydroglucose unit. The methyl substitution is stated, for example, as DS (M). Usually, the hydroxyalkyl substitution is described by the MS. The MS is the average number of moles of the etherifying reagent which form ether-like bonds per mole of anhydroglucose unit. The etherification with the etherifying reagent ethylene oxide is stated, for example, as MS (HE), and the etherification with the etherifying reagent propylene oxide is stated as MS (HP). The determination of the side groups is effected by the Zeisel method (literature: G. Bartelmus and R. Ketterer, Z. Anal. Chem. 286 (1977) 161-190).
Various properties of the products are established via the degree of etherification and the type of substituents, such as, for example, the thermal flocculation point, the solubility, the viscosity, the film formation capacity, the water retention capacity and the adhesive strength. MC and MHAC are used in different fields of use, for example as consistency regulators and processing aids in mineral and dispersion-based construction material systems or in the production of cosmetic and pharmaceutical compositions. Cellulose ethers having high degrees of substitution are also suitable as thickeners for organic solvents.
An overview of the chemical fundamentals and principles of production (production processes and process steps) and a list of substances and description of the properties and potential uses of the various derivatives is disclosed, for example, in Houben-Weyl, Methoden der Organischen Chemie, Makromolekulare Stoffe [Methods of Organic Chemistry, Macromolecular Substances], 4th edition, volume E 20, page 2042 (1987).
Although cellulose ethers as an end product with an appropriate degree of substitution are usually soluble in water at a temperature below the dissolution temperature, a small insoluble fraction is usually retained, which causes turbidity of the solution. This turbidity is primarily caused by insoluble cellulose fibres which were not sufficiently activated by the alkali metal hydroxide and have acquired a lower degree of substitution. The magnitude of the turbidity and of the proportion of undissolved fibres has an adverse effect on the usability of the cellulose ethers for certain fields of use. For example, the use of cellulose ethers for the production of transparent hard capsules in the pharmaceutical sector may be mentioned here. A low fibre content is also required for the use of cellulose ethers in ceramic extrusion for automotive catalytic converters, in order to avoid possible material defects. Very efficient and uniform alkalization is highly desirable for the production of a homogeneously substituted cellulose ether having a small insoluble fibre fraction.
In addition, the coupling of low pressure and high pressure process steps results in increased equipment requirements. Usually, pressure-resistant horizontal, cylindrical drums equipped with a horizontal central shaft with specific mixing elements, such as ploughshares, Becker blades, T-blades, paddles, mixing spirals or mixing ribbons, are used in the production of cellulose ethers for the alkalization and/or the heterogeneous etherification. Examples for such mixing reactors which follow the principle of throw or intensive mixers (rotor speeds are above the critical speed which means that particles are hurled out of the material bed) are DRUVATHERM® Reaktor DVT or All In One Reactor® TR from Gebr. Lödige Maschinenbau GmbH, Germany. In addition, some of the reaction mixers are equipped with shredders, commonly designated as choppers or cutting rotors in order to achieve an additional mixing effect or a good distribution of an additive. For effective cooling or heating, these mixers are typically provided with a double jacket or cooling/heating coils. Embodiments having attached condensers and external condensers are also known.
Regarding the optimization of alkalization, various approaches were adopted in the past. In this context, an improved distribution of the sodium hydroxide solution by premixing with an etherifying agent in a nozzle prior to mixing with the cellulose is reported (U.S. Pat. No. 4,845,206 A). Furthermore, improved alkalization upon addition of surface-active substances is described (U.S. Pat. No. 7,361,753). Low pressure spray alkalization takes place in these processes in pressure resistant reaction vessels. The use of an expensive and complex pressure resistant reactor for low pressure alkalization is undesirable when optimum use of apparatus and plant utilization is desired.
A number of patent applications have been published which are concerned with the subject of optimized alkalization for reducing the undissolved fibre content. Separate alkalization steps in suitable apparatuses are described. Described are the implementation of a slurry process for alkalization with downstream removal of alkali and the implementation of an upstream continuous spray alkalization in a very wide range of units. For example, bucket chain conveyors (US 2007/0149773 A1), kneaders (US 2002/0099203 A1), rotary pressure filters (US 2007/0144692 A1), high-intensity mixers (US 2006/0287518 A1) or screw conveyors (US 2007/0149773 A1) are used here. The uniform feed of sodium hydroxide solution to a constant and continuous flow of cellulose mass while ensuring thorough mixing is the aim of the continuous alkalization disclosed in these publications. The advantage of the decoupling of low pressure and high pressure process steps is not disclosed in these publications. In addition, the critical restrictions of continuously metering and ensuring a constant cellulose mass flow to achieve a constant and uniform concentration of sodium hydroxide in all parts of the cellulose are emphasized.
Other processes for the production of methyl hydroxyalkyl celluloses are described, inter alia, in US 2005/0240016 A1 and in U.S. Pat. No. 3,388,082 A, U.S. Pat. No. 4,456,751 A, U.S. Pat. No. 4,477,657 A, and U.S. Pat. No. 3,839,319 A.
Therefore, it is an object of the present invention to provide a process which optimizes the production of alkali cellulose. It is a preferred object of the present invention to provide a process ensuring homogeneous and/or uniform alkalization of the cellulose in order to obtain in the following derivatization process homogeneously substituted cellulose derivatives. Preferred derivatization processes are alkylating and/or hydroxyalkylating step(s) to produce homogeneously and/or uniformly substituted cellulose ethers.